i) Field of the Invention
A technique of this disclosure relates to an electronic component such as a voltage nonlinear resistor and relates to a technique for imparting flame retardancy or incombustibility to an exterior packaging material covering an element.
ii) Description of the Related Art
Various devices such as electronic devices and electric devices include a combustible material such as plastic used for a housing etc., for the purpose of weight saving and, additionally, since electronic components are mounted at higher density due to requirements for a reduced device size etc., a burnout of an electronic component causes damages of adjacent electronic component and an entire device.
A varistor (voltage nonlinear resistor) is used as an electronic component protecting electronic components and a device from such an inconvenience. The varistor has voltage nonlinear resistance characteristics in which resistance drastically decreases in response to a rise in applied voltage, and is therefore used as a surge absorption element.
In an example of the varistor, on the both surfaces of a sintered body acquired by molding of zinc oxide powder mixed with a trace amount of bismuth oxide powder etc. in a metal mold into a disk shape followed by sintering at 1000[° C.] or higher, disk-shaped electrodes with a diameter smaller than the sintered body are baked and, after lead wires are connected by solder to respective outer surfaces of the electrodes to form an element, this element is coated with an epoxy resin to form an exterior packaging. This exterior packaging fulfills a function of increasing the mechanical strength and heat resistance of the varistor.
The varistor is generally used for protecting electronic components and a device from an external or internal surge and, if the varistor absorbs a surge exceeding a limit of absorption energy, the varistor may be broken and put into a short-circuit state, resulting in the risk of combustion of an exterior packaging material thereof. The exterior packaging material of the varistor is typically made with an inorganic filler component and an epoxy resin component and the combustion of the exterior packaging material is attributable to combustion of the epoxy resin component.
Therefore, a flame-retardant material is used for the exterior packaging material. This flame-retardant material used is, for example, an epoxy resin containing bromine or antimony, which is a flame-retardant agent. However, although being flame-retardant, the epoxy resin may combust if the varistor generates heat in a continuous manner. Once the epoxy resin combusts, the combustion may continue until a combustible component in the exterior packaging material disappears.
It is known that this exterior packaging material is made incombustible by adding a flame-retardant agent that is bromine or antimony. When the flame-retardant agent is increased, the resin itself is reduced in a heated flow rate (fluidity), which makes it difficult to form an exterior packaging film. In the case of powder resin coating, the formation of an exterior packaging film becomes difficult at a resin amount of 30 [wt %] or less. If a combustible component in an exterior packaging material is reduced to a combustion limit amount or less, the exterior packaging material can be made incombustible.
A bromine-based flame-retardant agent, has a function of suppressing combustion of a resin component through gasification. A gasified bromine component imposes a heavy environmental burden such as ozone layer depletion and the usage thereof tends to be limited.
With regard to making a varistor exterior packaging material incombustible, a varistor is known that has a silicone resin used as a coating material excellent in incombustibility in a protective coat (e.g., Japanese Laid-Open Patent Publication No. 6-215910).
With regard to combustion suppression of a silicone resin, a varistor is known that is improved in flame retardancy of an exterior packaging material by adding aluminum hydroxide or magnesium hydroxide as a flame-retardant agent to a silicone resin or silicone elastomer so as to restrain ceramic contents and the exterior packaging material itself from scattering by rubber elasticity of the silicone resin or silicone elastomer (e.g., Japanese Laid-Open Patent Publication Mo. 2005-277100).
A varistor is also known that is coated by using as an exterior packaging material a silicone rubber acquired by adding a hardening agent to a liquid silicone main agent and adding aluminum hydroxide to these two agents (e.g., Japanese Laid-Open Patent Publication No. 2006-286986).